This invention relates generally to a pressure responsive shut-off device and, in particular, to a shut-off device utilized in the control of fluid operated motors.
Fluid operated tools, such as pneumatic nut setters, are in common use in assembly operations. A typical fluid powered nut setter is hand held by an operator, and is used to perform multiple operations such as setting a nut or other fastener precisely to a desired torque. In order to prevent reaction torque from being transferred to the operator as the nut is set and the motor stalls, such tools are typically provided with a shut-off valve intended to shut off the supply of motive fluid to the tool when the desired torque is reached. In such tools, it is well known that the torque produced by the motor is proportional to the motor back pressure. Therefore, such shut-off valves are commonly responsive to a predetermined motor back pressure to shut off motive fluid to the motor when the desired final torque is achieved.
One problem associated with shut-off valves used with fluid operated tools in which torque control is important relates to the wide variety of operations which can be encountered. For example, a pneumatic nut setting tool can be used for several types of fastening operations, including the fastening of joints referred to as "hard make" and "soft make". In a hard make, the nut or other fastener being rotated offers little resistance to the applied torque until it is finally set against a hard surface, at which time the resistance suddenly increases. Motor torque, therefore, is fairly constant throughout most of the cycle, then rises very steeply as the motor approaches or reaches a stall condition. In order to properly set the fastener, the valve must respond very rapidly to shut off the motive fluid.
As an example of a soft make, members being fastened may be pushed together by the nut as it is being tightened onto a threaded bolt. The motor torque will therefore increase more gradually, over a greater period of time, as the motor approaches a stall condition. In prior art shut-off devices, because of the different torque/time relationships encountered, the final torque actually realized at shut off varies widely between hard and soft makes.
An example of a fluid pressure responsive shut-off valve is disclosed in U.S. Pat. No. 3,373,824 to H. L. Whitehouse. In the Whitehouse device, a restrictive orifice is provided in the motive fluid flow path near the motor inlet. The orifice reduces fluid pressure at the motor inlet during free running of the motor. As the motor is loaded and approaches stall, the motor back pressure increases. A valve which is biased open by a spring is operable to interrupt the supply of motive fluid to the motor when the motor back pressure increases sufficiently to overcome the spring bias force and to close the valve.
U.S. Pat. No. 3,493,056 to J. H. Boeger et al. discloses a valve mechanism for shutting off the supply of motive fluid to a fluid motor which is compensated for variations in fluid supply pressure. The Boeger et al. valve includes a pilot valve element which shifts in response to an increase in motor back pressure above a predetermined proportion of the fluid supply pressure. A shut off valve responsive to the pilot valve then closes to shut off the supply of motive fluid to the motor.
U.S. Pat. No. 3,786,873 to Whitehouse discloses a stall torque regulator valve having a restricted orifice which is gradually closed by a sliding valve in response to an increase in motor torque. The orifice is so shaped that its effective size is reduced in a predetermined desired relationship to increasing torque as the orifice is closed by the valve sliding across the orifice.
Other illustrative pressure responsive fluid controlling devices are disclosed in U.S. Pat. Nos. 3,608,647 to Borries, 3,667,345 to Schaedler et al. and 3,696,834 to VonHoff, Jr.
In prior art shut-off valves of the character described above, mass and thus inertia of the valve components delay response of the valve to back pressure changes and thus delay valve component motion and closure of the valve. During this short interval, motor torque will continue to increase. This increase is generally small with soft make fasteners but can be substantial with hard make fasteners. The actual torque delivered when the valve is finally shut therefore is a function of the torque/time relationship of the associated assembly operation. This relationship between joint or fastener "make" and torque application, in practice, becomes simply torque application variability and cannot be tolerated.